In today's highly competitive marketplace, the ability to manage and track inventory is vitally important. A major cost to consumer retail stores and other businesses that handle a large inventory is the cost of tracking individual items of the inventory as those items move throughout the supply chain.
Traditionally, barcodes and barcode scanners have been used to track inventory. Barcode scanning systems work by labeling items with a barcode that encodes a product identification number. When needed, the barcode is read using a barcode reader. While this system is useful for many applications, barcodes have several drawbacks. First, barcodes are limited in the amount of information that can be encoded. Also, once a barcode is printed, it is difficult to change the barcode and thus it is difficult to change the encoded information. Additionally, a barcode must typically be in the line of sight of the barcode reader to be read.
To alleviate some of the drawbacks of barcode systems, various Radio Frequency Identification (RFID) systems have been proposed. In a typical asset-tracking embodiment, a RFID system comprises at least one RFID reader and at least one RFID tag. RFID tags are placed upon the asset to be tracked. RFID tags typically fall into one of two types; active RFID tags, which include an on-board power source (such as a battery) or passive RFID tags, which are powered by a radio frequency carrier wave sent from the RFID reader. Active RFID tags typically can be read by a RFID reader at a longer range than passive RFID tags, which typically must be near the tag reader in order to receive the carrier wave from the RFID reader to power the RFID tag.
Passive RFID tags typically store data in a non-volatile memory. To retrieve the stored data, a RFID reader emits a time varying radio frequency (RF) carrier wave, which powers the passive RFID tag by the generation of an AC voltage across the antenna of the passive tag. The AC voltage is typically rectified to a DC voltage. The DC voltage builds until a minimum operating DC voltage is reached, enabling the RFID tag. Once enabled, the RFID tag can send data stored in the RFID tag memory to the RFID reader. This is typically done by modulated backscattering of the carrier wave received from the RFID reader. The RFID tag backscatters by causing changes in the amplitude and/or phase of the RFID reader's carrier frequency. The RFID tag performs the modulation of the RF carrier wave by altering the load impedance of the RFID tag's antenna.
The antenna on a typical RFID tag is designed to receive a RF carrier wave at a particular frequency. However, various environmental factors can detune the RFID tag's antenna, resulting in a shifting in the frequency to which the RFID tag antenna is sensitive. For example, a RFID tag attached to a liquid filled container can experience antenna detuning due to a parasitic capacitance provided by the container. The amount of this detuning can vary as the package is moved; if the package tilts, less liquid may be near the RFID tag, resulting in a smaller parasitic capacitance and therefore, a smaller amount of detuning.
Therefore, there is a need to provide RFID tags that have a tunable antenna and an associated reader.